The development of the vertebrate nervous system requires switching of polymorphic ATP-dependent chromatin remodeling complexes from a neural progenitor complex (npBAF) to a neuron-specific complex (nBAF) at mitotic exit. This switch is in part directed by two microRNAs, miR-124 and miR-9, which when expressed in fibroblasts can convert them into functional neurons. Recent exome sequencing studies have found frequent mutations of BAF complex subunits in non-syndromic mental retardation, microcephally, schizophrenia and less frequent BAF complex mutations in sporadic autism and sporadic ALS. The apparently genetically dominant nature of these mutations is consistent with the instructive role of subunit switching during neurogenesis and suggests that these chromatin regulators may have rate-limiting functions. Relatively little is known of the underlying mechanisms directing subunit switching, or how subunit switching relates to other more well-defined programs of neural development. Also, the mechanisms underlying the contribution of subunit mutations to a diverse range of neurologic diseases in humans are unknown. Indeed most of the mutations are in subunits not required for conventional activities such as nucleosome remodeling. Our preliminary data suggest that repression of three progenitor subunits and their substitution with neuron-specific subunits leads to mitotic exit of neural progenitors (NPGs) and functional differentiation. We will test this hypothesis using genetic approaches and define the genetic circuitry involved in repression of the npBAF subunits and the activation of the nBAF subunits. We will also define the proteins that bind to the neuron- specific surfaces of the complexes that mediate their role in dendritic morphogenesis, dendritic targeting and neural fate determination. Using chromatin immunoprecipitation and sequencing we will define the genome- wide consequences, which result from the normal exchange of three subunits within these 2 mega dalton chromatin remodeling complexes. Our studies and those of others indicate that an important function of BAF complexes is to oppose Polycomb complexes; however the mechanisms underlying this opposition are not understood. Thus, we will use a method recently devised in our lab to understand the underlying mechanism of BAF-Polycomb opposition. At the conclusion of our studies we hope to have an understanding of the fundamental genetic circuitry and mechanisms critical for this epigenetic switch and to gain insight into the roles of polymorphic BAF complexes in diverse human neurologic diseases.